Found: the biological clock with an alarm

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A test that shows how fast a woman's biological clock is ticking and how long she has left to start a family has been developed by British researchers.

The technique could revolutionise infertility treatment and identify the one in 10 women at risk of an early menopause.

It could also help women in their 30s to decide how long they can safely delay having children while they build a career.

The test uses an ultrasound scan to work out the size of a woman's ovaries and a complex computer program to predict how many eggs she has left.

Dr Hamish Wallace, a cancer specialist at the Royal Hospital for Sick Children in Edinburgh, who helped develop the technique, said: "In essence, it means that we now have the potential to be able to tell a woman how much time she has before her biological clock runs down."

Ovaries are thought to have a finite number of eggs and the number drops throughout a woman's life. When it reaches about 25,000 - typically around 37 years of age - the decline accelerates and over the next few years women find it more difficult to conceive. When the number reaches about 1000 the first signs of menopause begin to appear.

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Dr Wallace and Dr Thomas Kelsey, a computer scientist at St Andrews University, looked at research linking ovary size with declining fertility and applied a complex mathematical and computer model to the results.

Their findings are reported in the journal Human Reproduction.

Dr Wallace said: "We have shown that ovarian volume in women aged from 25 to 51... may be used to estimate accurately how many follicles (eggs) are left and therefore what is the woman's reproductive age."

Dr Kelsey said the technology for the technique already existed and was "readily available all over the Western world... It is possible that this service can be made available at doctors' surgeries or at fertility clinics, and would be a likely first step in the family planning scenario."

In Australia, the average age of menopause is 51, although it varies from woman to woman. Disruption to the menstrual cycle can start 10 years earlier. About 10 per cent of women go through the menopause far earlier and in rare cases can become infertile in their twenties. Family history can give a clue, but there is no reliable way of predicting the loss of fertility.

The test is unlikely to be widely available for some time. The researchers are initially using the technique on young women whose chances of having children have been reduced by cancer treatment. They are also monitoring healthy young women to see if the technique gives accurate results.